This invention relates generally to information recording media and, more particularly, to recording media of the type in which information can be recorded and read by optical techniques.
Information recording media of this particular type are especially adapted for use as discs for storing frequency-modulated video signals and digital data. In the past, record discs of this type have typically comprised thin metallic films that are coated onto glass substrates using standard vacuum techniques. Information has typically been recorded in such a disc by a thermal process in which an intensity-modulated beam of radiation is focused onto the disc as the disc is rotated with respect to the beam, whereby a series of microscopic holes are melted in the metal film. A more detailed discussion of this technique for melting holes in a metallic recording medium is set forth in a co-pending and commonly assigned application for U.S. Pat. Ser. No. 890,407, filed in the name of John S. Winslow and entitled "Mastering Machine".
Metallic film recording media have not proven entirely satisfactory, however, because most of the energy in the beam of radiation is lost due to reflection off the outer surface of the film and transmission into the substrate, and because the metallic film ordinarily has a relatively high thermal conductivity and a relatively low melting temperature. This latter factor results in a conduction of the heat generated by the absorption of the beam of radiation radially outwardly from the point of impingement, which thus forms a hole in the film significantly larger than the cross-sectional size of the beam. This limits the density of information that can be stored on the recorded medium. Additionally, melting of the metal film usually results in a metallic residue being deposited around the periphery of each pit, thereby reducing the signal-to-noise ratio of the signal that can be reproduced from the medium.
An improved information recording medium is described in U.S. Pat. No. 4,097,895, issued in the name of F. W. Spong and entitled "Multilayer Optical Record". The record medium described therein includes a layer of highly light-absorptive organic dye material overlaying a substrate having an upper light-reflective surface. The thickness of the light-absorptive layer is selected to establish an anti-reflection condition for an intensity-modulated beam of light being directed at it, whereby the energy in the light beam is coupled into the material with a higher optical efficiency than was previously the case with metallic film recording media. A series of holes is thus evaporated in the dye material, with a minimum light beam intensity. Organic dye materials typically have a relatively low thermal conductivity, so the successive holes formed by the intensity-modulated light beam are relatively small, and a relatively high recording density can be achieved.
Although the aforementioned multilayer optical recording medium can be used advantageously in some situations, there is still a need for a recording medium wherein energy can be coupled from a beam of radiation to the medium with even greater optical efficiency, especially for light-absorptive materials having relatively low indices of absorption, and especially when the light-absorptive layer has sufficient thickness to permit use of the record medium as a master for use in replication. The present invention fulfills this need.